This application relates to unthreaded elastomeric fasteners adapted to be secured to a relatively rigid screw threaded member and in one embodiment comprises a protective cap of the type adapted to be screwed on an exposed screw shank or threaded stud extending through a screw hole in a panel, thereby to secure the shank in place and where desired, to provide a fluid seal with the panel around the screw hole and also to protect the hands of personnel against injury from sharp portions of the stud or shank.
In some applications, the screw-threaded stud comprises a headless shank having a portion molded within a plastic element adapted to be secured to the panel by the cap screwed on a remaining portion of the shank extending through a hole in the panel. The screw-on torque and final binding or thread lock between the cap and shank must be sufficient to assure retention of the cap and to prevent its unplanned loosening and removal from the shank, yet obviously must not be so large that the shank is pulled loose from its mounting within the element when the cap is tightened to its final condition, nor so large that the screw-off torque will cause the shank to damage its mounting or to be unscrewed therefrom, especially in the event the mounting merely comprises a molded plastic element having one end of a headless shank frictionally retained or bonded therein.
One of the problems involved in the clamping of two members together by screw-threaded means has been to obtain a predetermined clamp load on the basis of various controllables, such as the screw-on torque, the size of the bolt hole, the diameter of the fastener, the grade and pitch of the screw thread, etc. In actual practice, however, uncontrolled variables creep in, such as small dimensional variations between the bolts, burrs and poorly formed or galled screw threads, frictional variations, cross-threading, and the substitution of metric fasteners for approximately comparable U.S. fasteners, all of which conspire to effect an actual clamping force sometimes much different from the desired force, even when the screwing torque is carefully controlled.
Important objects of the invention are to provide an improved cap of the foregoing type, or cover for an exposed screw shank, and a method for using such a cap, wherein a particularly compact, simple and inexpensive cap achieves a positive seal with a panel around the screw hole, assures an enhanced thread lock between the bore of the cap and the shank, provides particularly effective means for rapidly increasing and eventually limiting the screwing torque to a maximum allowable torque when the cap is screwed to its final tightened condition on the shank, and likewise assures a desired clamping force at said final condition, thereby to prevent damage to the threaded shank or parts secured thereto.
Other objects are to provide an improved cap of the foregoing type and method of using the same which renders the screw threads of all bolts essentially comparable in clamping effectiveness to class A threads when the bolts are manufactured within reasonable tolerances, thereby to effect a dependable relationship between the final tightening torque and the clamping force of the bolt and enabling a desired clamping force to be readily achieved merely by controlling the maximum allowable screwing torque.
Other objects are to provide such a cap that may be readily molded from an elastomeric polyurethane or comparable material preselected to provide a smooth unthreaded shank receiving bore adapted to receive the threaded shank with an interference fit and to screw thereon against the panel, whereby the elastomeric material of the cap deforms resiliently within its elastic limits into the shank threads to effect the screw action and at the predetermined maximum allowable torque merely flows within its elastic limits over the screw threads from one helical turn to the next to terminate the screw action without being cut or ruptured, and whereby the elastomeric material thereafter returns to its initial smooth-bore unthreaded condition for reuse as before in the event the cap is unscrewed from the shank.
Other objects are to provide such a cap and method of use whereby the torque required to screw the cap on the shank is determined by such factors as the stiffness, elasticity, friction, and the like, of the elastomeric material, the interference fit between the bore and shank, and the pitch and dimensions of the shank threads, which factors determine the force required to displace the polyurethane or comparable elastomeric material of the cap during the screw action; and whereby, as the cap is tightened against the panel and the axial or clamping force on the bolt shank increases, the cap material is resiliently stressed within its elastic limits by the clamping force and displaced axially along the screw threads of the bolt shank, thereby to elongate the portions of the cap adjacent the bolt threads and simultaneously to reduce the effective diameter of the bore of the cap. In consequence, as the cap is tightened to its final condition, the bore of the cap is constricted around the bolt shank to rapidly accelerate the screwing torque and the clamping or axial force on the bolt until the aforesaid predetermined maximum torque is attained and the elastomeric cap material flows over the screw threads without increasing the clamping force.
The screw-tightening torque must overcome the resilient reaction of the elastomeric cap material as it is displaced into the thread grooves of the shank. No such force is required to deform the elastomeric material during the unscrewing. In fact, the resiliency of the elastomer attempting to return to its undeformed condition aids the unscrewing. By suitably determining the above-noted factors that determine the maximum screw-tightening torque and clamping actions to be effected by the cap, the related and smaller unscrewing torque can also be essentially determined. Thus, a desired differential between the maximum tightening and loosening torque may be preselected such that the loosening torque may be any desired amount between 50% to 75%, for example, of the maximum tightening torque.
As a result of the structure described, a 35 to 40 in. lb. torque may be applied by a power tool to tighten the cap to its final condition. Because the resiliency of the elastomeric cap material displaced axially along the screw threads by the torque required to tighten the cap exerts a major force attempting to unscrew the cap, a few additional inch-pounds of unscrewing torque will release the cap to return to its axially unstressed condition and thereby significantly reduce the binding action of the cap on the shank, enabling an ordinary man to unscrew the cap manually in the field by exerting, for example, merely 25 in. lbs. of torque. Such a facility is particularly desirable where the assembly secured to the panel by the screw shank and cap requires frequent replacement of parts, as for example a light bulb in an automobile taillight assembly. After a defective light bulb is replaced, the cap may be screwed on the shank manually to secure the taillight assembly to the panel temporarily until effective tools are available.
Also, by using a polyurethane elastomer for the cap, the "break-force" comprising the initial force required to loosen the cap from its final tightened position on the shank and against the panel may be maintained comparatively constant with time because the polyurethane does not harden or lose its elasticity appreciably with age. By virtue of the unthreaded bore, the same cap may be used with a limited range of different screw sizes and thread standards, as for example with both U.S. and metric threads.
Another and more specific object of the invention is to provide a cap as described above comprising an elastomeric polyurethane or comparable material formed with a thin flexible annular sealing lip having a comparatively small area for abutting the panel surface and flexing to conform to minor irregularities therein and to provide an effective seal around the bore when the cap is screwed to its final position on the shank.
One of the problems involved in the use of such a cap is that an effective seal is frequently achieved between the flexible lip and panel appreciably before the axial reaction force of the sealing lip against the panel is sufficient to stop the screw action. In some instances, the sealing lip, especially where a particularly soft or flexible lip is desired, will be deformed to the extent that the seal is impaired. In any event, where a soft or especially flexible sealing lip is desired, the lip does not provide positive means for limiting the screw induced axial travel of the cap toward the panel. In consequence, the axial relationship between the cap and panel will vary from cap to cap unless the specifications for the cap and screw are maintained within extremely close and costly tolerances. Variations in the various physical or elastomeric characteristics of the polyurethane from one batch to another are sufficient in themselves to render it difficult to predetermine the axial relationship between the cap and panel when a maximum allowable screwing torque is applied.
Other objects are therefore to provide means on the cap for precisely determining the axial relationship between the panel and cap, regardless of the flexibility of the sealing lip, when a predetermined maximum allowable torque is applied to screw the cap against the panel; and specifically to provide an annular boss or abutment of the cap in addition to the sealing lip, wherein the abutment has a comparatively broad abutment surface engageable with the panel when the desired axial relationship between the panel and cap is obtained by the screw action, as for example as soon as the seal between the sealing lip and panel is assured.
In accordance with the structure described, as soon as the abutment surface engages the panel, the resilient reaction to screw induced travel of the cap increases rapidly and the maximum allowable screwing torque, whereat the elastomeric material of the cap merely flows across the screw threads, is achieved almost immediately thereafter.
Still another and more specific object is to provide essentially a reversal of the parts described above wherein instead of screwing an elastomeric cap having a smooth cylindrical bore on a threaded shank, a circular cylindrical extension of an elastomeric member is screwed into an internally threaded bore or well, whereby essentially the same achievements as described above are obtained, including the maximum allowable screwing torque, the axial clamping force and radial thread-lock force as direct and reliable functions of the maximum screwing torque, regardless of typical dimensional variations between the parts and imperfections in the screw thread.
Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate the corresponding parts in the several views.